Magnetic bearing drive circuit

Abstract

There is provided a magnetic bearing drive circuit driven from a pair of Direct Current (DC) link voltage rails, said magnetic bearing drive circuit having at least one amplifier providing an output PWM drive signal for driving a magnetic bearing winding, said output PWM drive signal being provided to the magnetic bearing winding through a plurality of drive signal rails, said magnetic bearing drive circuit comprising a pair of voltage offset devices coupled to the DC link voltage rails and arranged to provide a pair of offset catcher voltage rails from the DC link voltage rails, and primary clamping means coupled between each offset catcher voltage rail and a respective one of the drive signal rails.

The invention claimed is: 
     
       1. A magnetic bearing drive circuit driven from a pair of Direct Current (DC) link voltage rails, said magnetic bearing drive circuit having at least one amplifier providing an output Pulse Width Modulation (PWM) drive signal for driving a magnetic bearing winding, said output PWM drive signal being provided to the magnetic bearing winding through a plurality of drive signal rails, said magnetic bearing drive circuit comprising:
 a pair of voltage offset devices coupled to the DC link voltage rails and arranged to provide a pair of offset catcher voltage rails from the DC link voltage rails; and 
 primary clamping circuitry coupled between each offset catcher voltage rail and a respective one of the drive signal rails. 
 
     
     
       2. The magnetic bearing drive circuit of  claim 1 , wherein each voltage offset device further comprises a decoupling capacitor network connected between earth and each offset catcher voltage rail. 
     
     
       3. The magnetic bearing drive circuit of  claim 1 , wherein the primary clamping circuitry comprises a pair of clamping diodes per respective drive signal rail. 
     
     
       4. The magnetic bearing drive circuit of  claim 3 , wherein the pair of primary clamping diodes are coupled in series between the offset catcher voltage rails, and the junction between the primary clamping diodes is coupled to the respective drive signal rail. 
     
     
       5. The magnetic bearing drive circuit of  claim 1 , wherein the voltage offset device comprises:
 a series resistor coupled between the respective DC link voltage rail and offset catcher voltage rail; or 
 a (power) Zener diode having a predetermined threshold voltage coupled between the respective DC link voltage rail and offset catcher voltage rail; or 
 a DC-DC converter, whose output is the DC link voltage between the DC link voltage rails and whose input is a predetermined offset voltage, 
 coupled between the respective DC link voltage rail and both offset catcher voltage rails; or 
 a capacitor, resistor and switch forming a brake circuit. 
 
     
     
       6. The magnetic bearing drive circuit of  claim 5 , wherein the voltage offset device comprises a Zener diode, and the circuit further comprises a resistor in series with each Zener diode. 
     
     
       7. The magnetic bearing drive circuit of  claim 5 , wherein the voltage offset device comprises a power Zener diode formed from a low power Zener diode coupled across a power Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) or power insulated Gate Bipolar Transistor (IGBT). 
     
     
       8. The magnetic bearing drive circuit of  claim 7 , wherein the power Zener diode comprises a plurality of low power Zener diodes selectively coupled across a power MOSFET or power IGBT, to provide a variable threshold voltage power Zener diode. 
     
     
       9. The magnetic bearing drive circuit of  claim 1 , further comprising rectifier diodes coupled between the voltage offset devices and the DC link voltage rails to ensure power only transfers back to the DC link voltage. 
     
     
       10. The magnetic bearing drive circuit of  claim 1 , wherein the voltage offset devices are positioned:
 at the amplifier end of the overall system comprising the magnetic bearing, magnetic bearing amplifier and length of cables there between; or 
 at the magnetic bearing end of the overall system comprising the magnetic bearing, magnetic bearing amplifier and length of cables there between. 
 
     
     
       11. The magnetic bearing drive circuit of  claim 1 , further comprising secondary clamping circuitry coupled between each DC link voltage rail and drive signal rail. 
     
     
       12. The magnetic bearing drive circuit of  claim 11 , wherein the secondary clamping circuitry comprises a further set of (power) diodes in series with resistors coupled between each DC link voltage rail and drive signal rail.